Coating compositions are applied to substrates for many reasons, as for example to modify the color, brightness, whiteness, smoothness, gloss and/or porosity of the substrate. They can also be used to improve the ability of the surface of the substrate to receive additional coatings, ink and the like, and to increase the stiffness, strength, and/or weight of the substrate.
One problem with conventional roll applied coatings is incomplete transfer of the composition from an applicator roller to the substrate which can ruin the coated substrate surface. Transfer can be improved by lowering the solids content of the composition but this is undesirable because the excess water utilized to lower the solids content must subsequently be removed. The excess water can also weaken the substrate and permit the coating to be wicked into the substrate rather than remain on the surface.
A high solids content is desirable so that a single, thick coating can be obtained. A high solids content can also cause inadequate flow and leveling of the composition which results in uneven coating thickness and can cause webs to be formed between the composition applicator and the substrate.
Conventional (non-associative) thickeners can be added to attempt to solve these problems but coating compositions containing such thickeners also have processing problems.
For example, conventional thickener-containing compositions can have a high extensional viscosity which causes misting/spattering or pattern formation that can ruin the coated substrate.
The high extensional viscosity may also result in high blade pressure when the conventional thickener-containing composition is applied by a blade coater. High blade pressure requires a slower line speed or results in paper breaks and/or scratching of the substrate. Furthermore, the high blade pressure can undesirably force water and coating from the substrate.
Non-ionic associative thickener-containing compositions also exhibit low extensional viscosity, but because they are lower in molecular weight they are absorbed into the substrate and thus do not remain on the surface to improve it.
Conventionally coated substrates typically are of relatively poor quality and do not have the desired gloss, uniformity of appearance and print density. These problems are particularly apparent in the production of paper of newsprint quality.
U.S. Pat. No. 4,514,552 to Shay et al. discloses alkali soluble latex thickeners suitable for use as associative thickeners. However, the Shay Patent does not disclose or suggest the use of these thickeners in a method of coating substrates.